A pretensioner is used with a vehicle seat belt to rapidly tension a length of seat belt webbing at the onset of a crash to more securely restrain the vehicle occupant by restricting his forward movement. This places the vehicle occupant in a more effective position for protection by a secondary restraint such as an airbag.
A modern pretensioner comprises a piston driven along a cylinder by gas from a pyrotechnically detonated gas generator. The piston is connected to a cable, that in turn is connected through a buckle to the webbing of a restraint retractor. On activation, the piston pulls the cable which in turn tensions the retractor webbing, pulling the webbing tight around the vehicle occupant. At the end of the pretensioning stroke it is necessary to stop the piston and prevent it moving back down the cylinder which would loosen the webbing again.
Various forms of locking mechanisms for the piston have been devised. An elliptical disc may be attached to the piston in such a way that it slides easily along the tube in a pretensioning direction but is tilted to jam into the sides of the tube in the reverse direction. This locks the piston against reverse movement.
Alternatively it has been proposed to place loose locking balls around a groove in the piston, on a tapering surface arranged such that the balls are located in the deepest part of the groove when the piston is moving in the pretensioning direction. However, when the piston is pulled back by the inertia of the occupant at the end of the pretensioning pulse, the balls slide to a shallower part of the groove and consequently engage the inner surface of the cylinder providing the requisite braking effect.
These prior art designs are expensive since they require many component parts that must be manufactured to close tolerances.